1. Field of the Invention
The present invention relates to sheets, strips, or films which undergo a visually observable color change in localized areas upon the application of pressure. The indicator can show that undesirable levels of pressure have occurred as by impact or tampering.
2. Background of the Art
It is highly desirable to have an indication of any violation of the integrity of a package or of any impact which might cause damage to the contents of a package.
Plastic wraps provide some indication of tampering, but evidence of tampering can be masked. Heat shrunk wraps are often applied to containers, but these can be softened, removed and replaced.
Under the lid devices have been used which change color upon rotation of the lid. These indicators generally have a transparent portion on the lid through which the color change can be viewed as shown in U.S. Pat. Nos. 4,480,760 and 4,519,515.
Packs for producing either useful reactions or chemical agents, having compartments of individual ingredients, at least one of which is a liquid, are constructed with rupturable seams between the compartments. The seams between the compartments are ruptured, mixing the ingredients still contained within the pack, to produce the desired effect, such as heat storage. Such a system is described in U.S. Pat. No. 4,462,224.